This invention relates to a wireless communication system and wireless communication method, and in particular relates to a wireless communication system and wireless communication method for transmitting and receiving frames comprising basic pilot symbols used for propagation path estimation, control symbols which convey control information necessary for data channel demodulation, and data symbols which convey information bits.
In order to increase the efficiency of data packet transmission in packet transmission for cellular mobile communication, adaptive modulation/demodulation control, adaptive spreading factor control, transmission power control, HARQ (Hybrid Automatic Repeat reQuest) control, scheduling control, and other types of adaptive wireless link control are employed. These types of control are performed using a control channel which is transmitted with the data channel; the transmitting station uses the control channel to notify the receiving station of the wireless link parameters being used in the data channel. For example, in the case of adaptive modulation/demodulation control, the control channel transmits the data channel modulation method (QPSK, 16QAM, or similar) and the coding rate. In the case of adaptive spreading factor control, the control channel transmits the spreading factor and spreading code, or the number of symbol repetitions and other information. In HARQ control, the control channel transmits the packet numbers transmitted over the data channel, the number of retransmissions, and other information. In the case of scheduling control, user IDs and other information is transmitted using the control channel.
FIG. 22 shows a frame configuration of the prior art, used in packet transmission for cellular mobile communication. One frame comprises basic pilot symbols SP, control symbols SC, and data symbols SD. In a downlink in which a base station performs transmission, the basic pilot symbols SP may be regarded as common pilot signals. Basic pilot symbols SP are employed for propagation path estimation which is necessary for demodulation of control channel and data channel signals. In the data channel, adaptive wireless link control is employed to execute control of the modulation method, coding rate, spreading factor, transmission power, and similar. These control parameters are conveyed by means of control symbols SC.
In adaptive wireless link control, control is executed to improve the data channel communication quality of a user experiencing poor communication quality at the edge of a cell, or to switch to a transmission method in which errors do not occur in an environment with poor communication quality, or similar. FIG. 23 is an example of a case in which transmission power control is performed for a user experiencing poor communication quality, at a cell edge or similar. As indicated in FIG. 23, in downlink communication control is executed so that the basic pilot symbol power is held constant, while the control symbol and data symbol transmission power is increased. In this way, in the example of FIG. 23, through transmission power control of the control channel and data channel, control is executed to improve communication quality; but adaptive control of basic pilot symbols is not performed. Hence ordinarily, in downlink communications, somewhat higher transmission power is allocated to the basic pilot symbols. However, at a cell edge there is the effect of interference from other cells, and so there is a tendency for channel estimation precision based on the pilot symbols to be degraded due to interference from other cells.
FIG. 24 is an example of a case in which adaptive spreading control is performed for a user at a cell edge or similar, experiencing poor communication quality. In this example, by increasing the number of repetitions of data symbols, the S/N of data channel is improved. Thus in the example of FIG. 24 also, control is executed to improve communication quality by controlling the number of symbol repetitions in the data channel, but adaptive control to improve the S/N of pilot symbols is not performed.
In this way, in conventional packet transmission methods the communication quality in the data channel between a user terminal at a cell edge or similar with poor communication quality and a base station is ensured through adaptive wireless link control, but there have existed no mechanisms for performing adaptive control of the pilot symbols, which greatly affect demodulation characteristics. Hence the estimation precision of channel estimation values used in data demodulation has not been improved, and so there has been the problem of a limit to improvement of the demodulation characteristics of the data channel.
In a first technology of the prior art, prescribed control symbols, such as for example the TFCI (Transport Format Combination Indicator) control symbols stipulated by 3GPP standards, are used as pilot symbols in addition to regular pilot symbols (see for example JP 2003-32146 A). By means of this first technology of the prior art, as a result of an increased number of symbols used as pilot symbols, the channel estimation precision can be improved.
As a second technology of the prior art, in a LAN format having a preamble portion and a payload portion, pilot symbols are inserted into the payload portion as well as into the preamble portion (see for example JP 2003-536288 A or WO01/059950 based upon PCT/US01/03778).
However, in the first technology of the prior art, the insertion of additional pilot symbols is not performed adaptively. And in the first technology of the prior art, the properties particular to the 3GPP standard are utilized, so that general application to channel estimation for wireless communication is not possible.
The second technology of the prior art is not a method of adaptive insertion of additional pilot symbols, but of always inserting pilot symbols into the preamble portion and into the payload portion, so that there is the problem that the number of pilot symbols increases and transmission efficiency is reduced.